1.1 Technical Field
The invention relates to a process for the polymerization of at least one polymerizable or copolymerizable monomer by the radical route in the presence of (1) a stable free radical and (2) a polymerization initiator.
1.2 Description of the Related Art
The presence of a stable free radical in a polymerization mixture makes it possible to control the growth of the polymer chains and generally to result in a polymer of low polydispersity, indeed with a polydispersity of approximately one, as is possible in anionic polymerization. However, the presence of the stable free radical is disadvantageous because it greatly slows down the reaction rate and makes it difficult, if not impossible, to obtain polymers with a high weight-average molecular weight, for example greater than 50,000 g/mol.
It is possible to attempt to overcome this disadvantage by adding a polymerization initiator to the polymerization. However, to date, the initiators used in mixtures comprising a polymerizable monomer and a stable free radical cause side reactions reflected by the partial destruction of the stable free radical and of the initiator itself. An additional difficulty is the formation of impurities which damage, in particular, the appearance of the final polymer. Due to these side reactions, the initiator has little effectiveness, lengthening the reaction time and making it difficult to obtain polymers with a high molecular weight. Moreover, the final polymer is colored and often opaque.
PCT Patent WO 94/11412 describes a polymerization process in the presence of a stable free radical and benzoyl peroxide as polymerization initiator. The polymers thus obtained exhibit a number-average molecular weight of less than 60,000 g/mol and a weight-average molecular weight of less than 70,000 g/mol and moreover exhibit a high yellowing index.
U.S. Pat. No. 4,581,429 describes a polymerization process which confers control on the formation of the growing polymer chains. This process requires the preparation of an alkoxyamine and results in oligomers with a molecular weight of less than 10,000 g/mol. This process also exhibits low rates of conversion of monomer to polymer.
The following references also disclose stable free radical and polymerization initiator systems: WO 95/26987, U.S. Pat. No. 5,412,047, U.S. Pat. No. 5,449,724 and BR 9400671. In these patent documents, the polymerization initiator is not regarded as an essential factor. In contrast, in the process according to the invention, the polymerization initiator is one of the novel features of the invention.
It has now been found that the use of a specific family of polymerization initiators makes it possible to overcome the above-mentioned disadvantages. The process according to the invention is fast, allows the initiator to act more effectively and results in polymers or copolymers exhibiting a low yellowing index, as determined according to ASTM standard D 1925.
The invention relates to a process comprising a stage of polymerization or copolymerization of at least one polymerizable or copolymerizable monomer by a radical route or mechanism in the presence of a stable free radical and of an initiator of formula:
R1xe2x80x94xe2x80x94Oxe2x80x94Oxe2x80x94R2
in which R1 and R2, which can be identical or different, represent hydrocarbon radicals comprising alkyl, such as aliphatics (straight and branched chain) and cycloaliphatics (substituted by alkyl and unsubstituted); aryl, such as aromatics; alkylaryl or aralkyl, i.e., mixed character groups. R1 and R2, independently, preferably comprise from 1 to 20 carbon atoms.
Preferably, at least one radical from R1 and R2 comprises an aromatic ring. The initiator preferably exhibits a half-life of one hour at a temperature of between 80xc2x0 C. and 150xc2x0 C. and, preferably, between 100xc2x0 C. and 130xc2x0 C.
Mention may be made, by way of example, of the following initiators:
di-tert-butyl peroxide,
tert-butyl cumyl peroxide,
dicumyl peroxide. Dicumyl peroxide is a particularly suitable initiator.
The initiator can be introduced into the polymerization or copolymerization mixture in the proportion of about 50 ppm to about 50,000 ppm by weight based on the total monomer weight.
The stable free radical should not be confused with free radicals with fleeting lifetimes (a few milliseconds), such as the free radicals resulting from the usual polymerization initiators, e.g., peroxides, hydroperoxides and initiators of azo type. Free radicals which are polymerization initiators tend to accelerate the polymerization. In contrast, stable free radicals generally tend to slow down the polymerization. Within the meaning of the present invention, a stable free radical is not a polymerization initiator and the mean lifetime of the stable free radical is at least five minutes under the conditions of use of the present invention. While not wishing to be bound by any particular theory, it is thought that during this mean lifetime, the molecules of the stable free radical continually alternate between the radical state and the state of a group bonded via a covalent bond to a polymer chain. Of course, it is preferable for the stable free radical to exhibit good stability throughout the duration of its use in the context of the present invention. Generally, a stable free radical can be isolated in the radical state at room temperature.
The family of the stable free radical materials includes compounds acting as radical polymerization inhibitors and stable nitroxide radicals, i.e., comprising the xe2x95x90Nxe2x88x92Oxe2x80xa2 group, such as the radicals represented by the following formulae: 
in which R1, R2, R3, R4, Rxe2x80x21 and Rxe2x80x22 which can be identical or different, represent a halogen atom, such as chlorine, bromine or iodine; a saturated or unsaturated, linear, branched or cyclic hydrocarbon group, such as an alkyl or phenyl radical; an ester group; an alkoxy group; a phosphonate group; or a polymer chain which can be, for example, a poly(methyl methacrylate) chain, a polybutadiene chain, a polyolefin chain, such as a polyethylene or polypropylene chain, but which is preferably a polystyrene chain, and in which R5, R6, R7, R8, R9 and R10, which can be identical or different, can be chosen from the same classes of groups just described for R1, R2, R3, R4, Rxe2x80x21 and Rxe2x80x22, and can furthermore represent a hydrogen atom; a hydroxyl group xe2x80x94OH; or an acid group such as xe2x80x94COOH, xe2x80x94PO(OH)2 or xe2x80x94SO3H, and in which n is preferably from 2 to 10 inclusive.
In particular, the stable free radical can be 2,2,5,5-tetramethyl-1-pyrrolidyloxy, sold under the trade name Proxyl, or 2,2,6,6-tetramethyl-1-piperidinyloxy, generally sold under the name Tempo.
The stable free radical can also be chosen from the following materials:
tert-butyl 1-phenyl-2-methylpropyl nitroxide,
tert-butyl 1-(2-naphthyl)-2-methylpropyl nitroxide,
tert-butyl 1-diethylphosphono-2,2-dimethyl propyl nitroxide,
tert-butyl 1-dibenzylphosphono-2,2-dimethylpropyl nitroxide,
phenyl 1-diethylphosphono-2,2-dimethylpropyl nitroxide,
phenyl 1-diethylphosphono-1-methylethyl nitroxide,
1-phenyl-2-methylpropyl 1-diethylphosphono-1-methylethyl nitroxide.
The stable free radical can be introduced into the polymerization or copolymerization mixture in the proportion of about 0.005% to about 5% by weight based on the sum of the weights of polymerizable monomer and of stable free radical.
The molar ratio of the stable free radical to the initiator is preferably between about 1.5 and about 2.5 and, more preferably, approximately 2.
In the context of the present invention, any monomer exhibiting a carbon-carbon double bond capable of polymerizing or copolymerizing by the radical route can be used.
At least one monomer present in the polymerization or copolymerization mixture can be a vinylaromatic monomer, an olefin, a diene, an acrylic monomer or a methacrylic monomer. The monomer can also be vinylidene difluoride or vinyl chloride.
The term xe2x80x9cvinylaromatic monomerxe2x80x9d is understood to include styrene; styrene substituted on the vinyl group by at least one alkyl group and/or halogenated alkyl group, such as a-methylstyrene or a-chloromethylstyrene; styrene substituted on the phenyl ring by at least one alkyl group and/or halogenated alkyl group, such as ortho-inyltoluene, para-vinyltoluene, ortho-ethylstyrene, 2,4-dimethylstyrene or 4-chloromethylstyrene; styrene substituted on the ring by at least one halogen, such as, for example, 2,4-dichlorostyrene; vinylanthracene; and para-acetoxystyrene.
The term xe2x80x9cdienexe2x80x9d is understood to mean a conjugated diene comprising from 4 to 8 carbon atoms, such as 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, piperylene and chloroprene.
The process according to the invention is particularly effective for vinylaromatic monomers and for diene monomers.
Apart from the distinctive characteristics of the present invention with regard to the initiator and the stable free radical, the radical polymerization or copolymerization is carried out under conditions known to a person skilled in the art, taking into account the particular monomer or monomers under polymerization or copolymerization.
In an embodiment where the monomer mixture comprises a vinylaromatic monomer and where excellent control of the growth of the polymer or copolymer is desired so that a particularly narrow polydispersity is achieved, it is preferable to carry out the polymerization or copolymerization at a temperature at which no polymerization or copolymerization is observed in the absence of the initiator. For example, in the case of the polymerization or copolymerization of at least one vinylaromatic monomer, this condition is realized when the temperature is less than approximately 120xc2x0 C. Thus, when the mixture comprises a vinylaromatic monomer, it is possible to carry out the polymerization or copolymerization between about 50xc2x0 C. and about 120xc2x0 C. Appreciable polymerization or copolymerization rates are nevertheless obtained by the process of the invention when the temperature is between about 90xc2x0 C. and about 120xc2x0 C.
Nevertheless, if a higher polydispersity is acceptable, it is possible to heat the mixture to higher temperatures. Thus, it is also possible to polymerize or copolymerize at temperatures of up to about 200xc2x0 C. if a greater rate of polymerization is preferred to the detriment of the polydispersity.
The process according to the invention can be carried out between about 50xc2x0 C. and about 200xc2x0 C. and, preferably, between about 100xc2x0 C. and about 130xc2x0 C.
In an alternative embodiment, the polymer or copolymer may be an impact vinylaromatic polymer, in which case the polymerization or copolymerization mixture generally comprises at least one vinylaromatic monomer and a rubber, the latter generally formed from at least one conjugated polydiene, such as polybutadiene formed from at least one of the isomers of butadiene.
The invention also relates to the preparation of copolymers. For example, when at least one vinylaromatic monomer is present in the mixture, this monomer can be copolymerized with, for example, at least one monomer chosen from the group of consisting of acrylonitrile, methacrylonitrile, acrylic acid, methacrylic acid, an alkyl ester in which the alkyl group contains from 1 to 4 carbon atoms, an N-alkylmaleimide in which the alkyl group contains from 1 to 4 carbon atoms, and N-phenylmaleimide.
The process according to the invention makes possible the preparation of block copolymers. Indeed, the polymerization of a first monomer in the presence of a stable free radical results in a living polymer first block. It is then possible to attach to this first block a second block of another polymer by placing the first living polymer block in a mixture of a second monomer, where further polymerization occurs. Thusly, it is possible to produce block copolymers, for example, copolymers comprising one or a number of polystyrene blocks and one or a number of polybutadiene blocks. The preparation of such block copolymers by the radical route requires good control of the polymerization of each of the blocks. Indeed, if a termination reaction interrupts the polymerization of a first block, it will not be possible to attach a second block of another monomer to the first block. The termination reactions must thus be as infrequent as possible. There are fewer termination reactions when, during the polymerization, the number-average molecular weight is more closely linearly proportional to the percentage of conversion. The existence of termination reactions is reflected by a decrease in the rate of increase in the number-average molecular weight as a function of the percentage of conversion.
The process according to the invention is particularly suited to polymerization or copolymerization in the organic phase, i.e., substantially in the absence of an aqueous phase, as is the case in bulk polymerization processes or solution polymerization processes in an organic solvent.
Of course, depending on the polymerization or copolymerization conditions, for example, the duration, the temperature and the degree of conversion of monomer to polymer or copolymer, it is possible to prepare products of very different molecular weight. The invention relates to the preparation of oligomers, polymers or copolymers with a weight-average molecular weight of less than about 10,000 g/mol and to the preparation of polymers or copolymers with a weight-average molecular weight of greater than about 10,000 g/mol, such as high polymers with a weight-average molecular weight generally ranging from about 100,000 g/mol to about 400,000 g/mol.
The invention relates to polymerization or copolymerization processes in which the degree of conversion of monomer to polymer or to copolymer is less than about 50% and to those in which the degree of conversion of monomer to polymer or to copolymer is greater than about 50%. For example, the degree of conversion of monomer to polymer or to copolymer can exceed about 60% and generally ranges from about 65% to about 80%.
The process according to the invention results in a polymer, copolymer or oligomer with a polydispersity generally of less than about 1.9 and with a yellowing index, measured according to ASTM standard D 1925, of generally less than about 20, preferably less than about 10 and more preferably less than about 5.
When the molar ratio of the amount of stable free radical to the amount of monomer is greater than about 1.2, the polydispersity of the final polymer, copolymer or oligomer is generally less than about 1.5.
The following characteristics are used in the examples:
The % conversion is the weight % of monomer converted to polymer. It was determined by 1H NMR on a Bruker 200 MHz device by integrating the NMR peaks corresponding to the polymer and to the monomer.
Number-average molecular weight (hereafter xe2x80x9cMnxe2x80x9d) is determined by gel permeation chromatography (GPC) in tetrahydrofurane at 30xc2x0 C., after calibration with standard polystyrene samples of known number-average molecular weight.
The polydispersity index (hereafter xe2x80x9cPIxe2x80x9d) is the ratio of the weight-average molecular weight to the number-average molecular weight, both measured by GPC (i.e., see Mn above).
The efficiency is equal to the ratio of the theoretical number-average molecular weight to the experimentally determined number-average molecular weight. The theoretical number-average molecular weight is given by the formula:       Mn    th    =                              (          M          )                s            -              (        M        )                    2      ⁡              [                                            (              I              )                        s                    -                      (            I            )                          ]            
in which (M), (M)s, (I) and (I)s represent the molar concentrations (in mol per liter) of monomer, starting monomer, initiator and starting initiator, respectively.
The concentration of initiator (I) as a function of the time (t) is determined by the equation:
(I)=(I)sexe2x88x92Kdxc2x7t
in which Kd represents the rate constant for decomposition of the initiator at the temperature under consideration.
The closer that the efficiency value approaches 1, the more effective the initiator and the stable free radical are in acting to control the polymerization and the less they participate in the undesirable side reactions, which partially destroy them and reduce their effectiveness.